1. Field of the Invention
The present invention relates to a liquid discharge head which discharges a liquid, a cleaning method for the liquid discharge head, a liquid discharge apparatus, and a substrate for the liquid discharge head.
2. Description of the Related Art
An ink jet head as a typical example of a liquid discharge head includes a plurality of discharge ports for discharging ink, a flow path in communication with the plurality of discharge ports, and an electro-thermal conversion element serving as a thermal energy generation element that generates thermal energy for causing the ink to discharge. The electro-thermal conversion element includes a heat generating resistor, and an electrode for supplying power thereto. An insulation protection layer having electrical insulation properties covers the electro-thermal conversion element, thereby ensuring insulation between the ink and the electro-thermal conversion element. The electro-thermal conversion element is driven to generate thermal energy that rapidly heats the ink to generate bubbles at an ink contact portion (thermal action portion) located above the electro-thermal conversion element, so that the ink is discharged and recording can be executed on a recording medium.
At that time, the thermal action portion of the ink jet head is affected by physical action such as impact arising from cavitation caused by generation and contraction of bubbles, or chemical action caused by the ink. In order to protect the electro-thermal conversion element from the above-described effects, the electro-thermal conversion element is provided with an upper protection layer (upper protection film) made of a metallic material such as a tantalum (Ta) film or a platinum group film (i.e., iridium (Ir) or ruthenium (Ru) film), each of which has relatively strong resistance to the impact arising from the cavitation and the chemical action caused by the ink.
In the thermal action portion which is a contact portion with the ink, a phenomenon occurs in which an additive substance such as a color material included in the ink is decomposed by being heated at high temperature to turn into a low soluble substance and then physically adsorbed onto a surface of the upper protection layer. The above phenomenon is known as “kogation”. If a kogation is formed on the thermal action portion of the upper protection layer as described above, thermal conduction from the thermal action portion to the ink becomes uneven, and generation of bubbles becomes unstable. Therefore, the ink discharge characteristics may be affected thereby.
As a method for solving the above issue, Japanese Patent Application Laid-Open No. 2008-105364 discusses a method for removing a kogation from the thermal action portion, in which an upper protection layer is provided to serve as an electrode for causing an electro-chemical reaction with ink so that a surface of the upper protection layer is eluted into the ink by the electro-chemical reaction. Specifically, in order to remove the kogation, a voltage is applied in such a way that the upper protection layer having the thermal action portion serves as an anode electrode, while an electrode arranged in a region other than the region of the thermal action portion within the same ink flow path serves as a cathode electrode.